1. Field of the Invention
This invention relates to microwave frequency discriminators.
2. Description of the Prior Art
Techniques and devices for the rapid and accurate determination of an unknown signal frequency are of significant interest in modern communication systems, in particular, for electronic counter measure (ECM) systems operating at microwave frequencies. Microwave frequency discriminators capable of converting incoming unknown frequencies into voltages for processing are often used in ECM systems. A microwave frequency discriminator may be defined as a circuit that provides an output voltage which is a predetermined function of and usually proportional to the frequency of an incoming signal. The discriminator voltage output versus frequency response, commonly termed the "discriminator characteristic," is the response in which the output voltage varies nearly linearly with respect to frequency over a predetermined frequency bandwidth. The bandwidth is generally determined by the slope, linearity, and resolution of the discriminator and is the frequency range over which the discriminator provides an unambiguous voltage output which is related to the input frequency.
A typical prior art broadband microwave discriminator such as shown by FIG. 1 utilizes passive elements such as transmission lines or an arrangement of lumped elements to vary the power level of the input signal applied to a detector diode as a function of frequency. Such a discriminator formed of various passive components as shown in FIG. 1 generally, disadvantageously, has a large number of connections between the components. Such interconnections of the components within the discriminator often produce impedance mismatches resulting in undesirable inflection points in the relation between input frequency and output voltage such that a certain voltage can occur at several frequencies. Such distortions in the discriminator characteristic curve limit the broadband resolution of the discriminator minimizing thereby the accuracy of the system.
Most prior art discriminators are preceeded with a limiter to provide a constant power input to the discriminator, whereby the output voltage of the discriminator is a function of frequency alone. An interface between the limiter and the discriminator further complicates the discriminator response and typically results in worse linearity. Limiters may not be required, however, if the incoming signal has a relatively constant magnitude. One of the problems introduced by the limiter is that the power output is not always ideally constant with frequency. In particular, a fall-off of the voltage at the end of the frequency band of the limiter combined with the linear frequency-voltage slope of the discriminator changes the overall discriminator characteristic resulting in an undesirably ambiguous frequency-voltage relationship.
As an example of a commonly used prior art passive element discriminator, attention is directed to FIG. 1 of the drawing. Discriminator 10 comprises a limiter 12 which receives an incoming RF signal 14 of unknown frequency and variable power level and converts signal 14 to an RF signal 16 of constant power level. Signal 16 is applied to a 3 db hybrid coupler 18 which splits signal 16 into two signal components. One component of the split signal is transmitted through a short path 20 and the other component through a longer path 22. The different path lengths produce different phase shifts of the two signal components. The split signals are recombined in another hybrid coupler 24. The power levels at the two outputs of the 3 db hybrid coupler 24 vary with frequency as a result of the vector summation of the two split signals having a differential phase shift. Detector diodes 26 and 28 are utilized to demodulate the frequency dependent signal received from coupler 24 and convert the signal into a dc voltage. A video amplifier 30 may be used to sum or compare and amplify the dc signal for subsequent measurement or display.